A light emitting diode (LED) is a solid-state semiconductor element including at least a p-n junction. The p-n junction is formed between a p-type and an n-type semiconductor layers. When the p-n junction receives a suitable forward voltage, the holes of the p-type semiconductor layer and the electrons of the n-type semiconductor layer are combined to emit light. Generally, the region emitting light is called a light-emitting region.
The light emitted from the light-emitting region is forwarded omni-directionally. However, a user usually needs only the light forwarding to a specific direction. Consequently, a reflective layer or a mirror for reflecting a portion of the light is adopted. Besides, the difference of the refractive indices between the LED's material and environmental medium can result in total reflection of the light emitting to the boundary of the LED in a specific incident angle. In general, it is unavoidable for each kind of the reflective light mentioned above to travel through inside the LED.
Referring to FIG. 1A, a known LED 100 includes a substrate 110 and an epitaxy layer 130. The epitaxy layer 130 includes an active layer 131 which can emit light omni-directionally when receiving a forward voltage. A reflective layer 150 is formed between the epitaxy layer 130 and the substrate 110 to reflect the light from the active layer 131.
A first ray R1 emits to the upside of the LED 100. When the refractive index of the environmental medium is less than that of the LED 100 and the incident angle is larger than the critical angle, the first ray R1 can be reflected totally at the boundary of the LED 100 and then return to the inside thereof. When the first ray R1 passes the active layer 131, a portion of the first ray R1 is absorbed by the active layer 131, and the other portion of the first ray R1 that is not absorbed emits to the reflective layer 150 and is reflected upward to pass the active layer 131 again. Thus, the first ray R1 resonates in the epitaxy layer 130, passes the active layer 131 repetitiously, and then is absorbed gradually. Under the similar mechanism, a second ray R2 emitting to the downside of the LED 100 also resonates in the epitaxy layer 130, passes the active layer 131 repetitiously, and then is absorbed gradually.
Referring to FIG. 1B, it shows no reflective layer is formed between the substrate 110 and the epitaxy layer 130 of the LED 100, and the substrate 110 is transparent relative to the light emitted from the active layer 131. The downside of the substrate 110 can attach to a mirror (not shown here) or air only. If a third ray R3 reflected from the bottom of the substrate 110 emits to the lateral wall of the substrate 110 with an incident angle θI larger than a critical angle θC, it can be reflected into the epitaxy layer 130 and be absorbed by the active layer 131. As mentioned above, the third ray R3 could be reflected totally at the boundary of the epitaxy layer 130 and then return to the inside thereof. Moreover, it could resonate in the epitaxy layer 130, pass the active 131 repetitiously, and then be absorbed thereby. The light absorption in the active layer 131 reduces the light extraction efficiency of the LED 100 to some extent. Especially for small chip like 8 mil or 10 mil which has larger area ratio occupied by the pad, light can be reflected by the pad more easily and has higher proportion to be propagated inside the chip. Thus, a lot of light is absorbed by the epitaxy layer 130 when passing there or by the pad. The light extraction efficiency is reduced obviously.
Referring to FIG. 1C, LED includes a GaAs substrate 140 mounted on a transparent substrate 110, an epitaxy layer 130 located on the GaAs substrate 140, and a scattering mask 120 located on the epitaxy layer 130. A forth ray R4 from the epitaxy layer 130 emits sideward through a transparent resin 160. Because the thermal resistance of the transparent substrate 110 is usually higher, it is difficult for an LED to dissipate the heat.
In the aspect of the application of LED, for example, a back light unit (BLU) which is one of the main components of liquid crystal display (LCD) needs a light source with the characteristics of high brightness, low power consumption, thinness, and lightness. Beside the conventional Electro luminescence (EL), cold cathode fluorescent lamp (CCFL) and hot cathode fluorescent lamp (HCFL), LED is also one of the point light sources employed by the BLU.